Thanks all for your fast reply!
I have attached a picture: the power button is located in the middle.
Usually, when it works, there is light (flash light when on stand by).
This button has a long history of malfunction. Someone who knows some computer repair, temporary fix it ( I do not how) . This person is not available any more.
The problem came last evening, when I put my computer on "sleep".
Today, I tried to use it, but nothing came up, so I decided to unplug the power and then plug in again.
Since then, I can't get power on my computer.
What option do you think I could get?